Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, but can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a pathological anatomy from multiple angles, with the patient positioned so the pathological anatomy is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the pathological anatomy, but passes through a different area of healthy tissue on its way to the pathological anatomy. As a result, the cumulative radiation dose at the pathological anatomy is high and the average radiation dose to healthy tissue is low.
The term “radiotherapy” refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centigray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment) and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted.
As one example, atrial fibrillation is a medical condition characterized by an abnormally rapid and irregular heart rhythm, because of uncoordinated contractions of the atria (i.e. the upper chambers of the heart.) A normal, steady heart rhythm typically beats 60-80 times a minute. In cases of atrial fibrillation, the rate of atrial impulses can range from 300-600 beats per minute (bpm), and the resulting ventricular heartbeat is often as high as 150 bpm or above. One conventional treatment for atrial fibrillation includes an open heart procedure involving incisions and ablations of areas of the atria to block the re-entry pathways that cause atrial fibrillation. However, when performing the procedure using external beam radiation therapy, it is difficult to track tumor motion that results from cardiac or respiratory motion, such as during treatment of atrial fibrillation.
In addition, there are many other medical applications where it is useful to accurately track the motion of a moving target region (pathological anatomy) in the human anatomy. For example, in radiosurgery, it is useful to accurately locate and track the motion of a target region, due to respiratory and other patient motions during the treatment.